Heart failure is a debilitating disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the tissues and organs of the body. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately fill with blood between heartbeats and the valves regulating blood flow become leaky, allowing regurgitation or back-flow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness and the inability to carry out daily tasks may result. Not all heart failure patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive. As heart failure progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds cardiac muscle causing the ventricles to grow in volume in an attempt to pump more blood with each heartbeat, i.e. to increase the stroke volume. This places a still higher demand on the heart's oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result, typically in the form of myocardial ischemia or myocardial infarction. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output. A particularly severe form of heart failure is congestive heart failure (CHF) wherein the weak pumping of the heart leads to build-up of fluids in the lungs and other organs and tissues.
In view of the potential severity of heart failure, it is highly desirable to detect its onset within a patient and to track its progression so that appropriate therapy can be provided. Many patients suffering heart failure already have pacemakers or ICDs implanted therein or are candidates for such devices. Accordingly, it is desirable to provide such devices with the capability to automatically detect and track heart failure and various techniques exploiting electrical impedance signals measured by an implantable device have been developed. Techniques exploiting impedance are presented, for example, in U.S. Pat. No. 7,505,814 to Bornzin et al., entitled “System and Method for Evaluating Heart Failure based on Ventricular End-Diastolic Volume using an Implantable Medical Device” and in U.S. Pat. No. 7,272,443 to Min et al., entitled “System and Method for Predicting a Heart Condition based on Impedance Values using an Implantable Medical Device.”
More recently, techniques for measuring impedance using hybrid impedance vectors were described in U.S. patent application Ser. No. 13/023,408, filed Feb. 8, 2011, of Min et al., entitled “Systems and Methods for Tracking Stroke Volume using Hybrid Impedance Configurations Employing a Multi-Pole Implantable Cardiac Lead”, which is fully incorporated by reference herein. In one example described therein, current is injected between a large and stable reference electrode and a right ventricular (RV) ring electrode. The reference electrode may be, e.g., a coil electrode implanted within the superior vena cava (SVC) or the device case or “can” electrode. Impedance values are then measured along a set of different sensing vectors between the reference electrode and the electrodes of a multi-pole left ventricular (LV) lead implanted via the coronary sinus (CS). These techniques are generally referred to as hybrid techniques since different vectors are employed for injecting current than for measuring the resulting impedance/voltage. More specifically, the techniques may be referred to as “LV-based hybrid techniques” since LV electrodes are used to measure the impedance. The LV-based hybrid techniques advantageously allow impedance signals to be detected that exhibit significant variation throughout individual cardiac cycles to aid in the detection of stroke volume and related cardiac function parameters and to aid in the optimization of pacing delays for use with CRT.
It would be desirable to provide hybrid impedance measurement techniques that additionally or alternatively exploit electrodes of a right atrial (RA) lead for measuring impedance values (based on current injected via the RV.) It is to these ends that various aspects of the present invention are directed.